It is known to cut windows in a sidewall of a main wellbore to drill offset or lateral wellbores from the main wellbore and for creating a pocket or window from which a core sample may be obtained.
The best-known and most widely used method for redirecting a drill bit off-center of a wellbore is by first setting a wedge-type device, known as a whipstock, by means of retaining it to the walls of the wellbore using slips and friction. A drill bit is then lowered and pushed to the side of the wellbore at the angle of the whipstock to start a sidetrack hole. Typically, windows cut using a whipstock may be rough and may present some difficulties when tying back the offset wellbore to the main wellbore's casing or liner. Typically, reaming a window using a whipstock requires multiple trips into the wellbore. Further, the full width and depth of the window can only be achieved at the bottom end of the whipstock.
During drilling of wellbores, conventional core samples are often taken to obtain information relative to the formations. Typically, coring occurs at the bottom of the wellbore during the process of deepening the hole. Typically, the process requires that the drill string be tripped out and a coring tool be run in for obtaining the core sample, after which the coring tool is removed and the drill string is run in to further deepen the borehole to total depth. The need for multiple trips into and out of the wellbore makes conventional coring time consuming and relatively expensive. Further, as the location for obtaining core samples is selected before drilling through a zone of interest, the formation cannot be assessed using well logging techniques and the like and therefore the core samples often have little or no value in assessing the wellbore.
Further, conventional wireline coring tools and technologies have imposed limitations regarding the retrieval of a useful length of continuous core, or can retrieve only very small samples of rock by means of trepan drilling or impacting perpendicularly into the wellbore wall.
Sidetrack coring tools form independent offset boreholes by projecting below a reaming collar or deflection tool. The coring tube may become trapped in the offset borehole and may not be retrievable therefrom. Further, other problems occur as a result of penetration of zonal interfaces without means for sealing the offset borehole and formation of short boreholes formed along a curved trajectory which compromise the ability to harvest a long, continuous, undisrupted core sample therefrom.
One form of coring assembly, set forth by Applicant in U.S. Pat. No. 5,103,921, suffers from some of the disadvantages of the prior art systems. A deflection crank at a lower end of a reaming and coring tube, contained within a reaming collar, and a universal ball joint at a top end of the reaming tube permit displacement of a lower end of the reaming tube for reaming a window into the main wellbore wall, after which the reaming tube projects below the reaming collar for cutting an angled offset borehole from which a core sample is obtained.
An improved, cost effective and reliable window reaming and coring apparatus, which is capable of cutting and retrieving long cores having a sizeable cross-section and which are substantially continuous and representative of the lithography of the main wellbore, is required. Further, the apparatus should be readily tripped in and out of the wellbore without risk of the apparatus becoming stuck during reaming or coring. Preferably, cutting of the core samples should occur after the wellbore has been drilled and logged to ensure that the samples taken represent zones of interest along the wellbore.